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Thinking in C++
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Constructor conversion

If you define a constructor that takes as its single argument an object (or reference) of another type, that constructor allows the compiler to perform an automatic type conversion. For example,

//: C12:AutomaticTypeConversion.cpp
// Type conversion constructor
class One {
  One() {}

class Two {
  Two(const One&) {}

void f(Two) {}

int main() {
  One one;
  f(one); // Wants a Two, has a One
} ///:~

When the compiler sees f( ) called with a One object, it looks at the declaration for f( ) and notices it wants a Two. Then it looks to see if there’s any way to get a Two from a One, and it finds the constructor Two::Two(One), which it quietly calls. The resulting Two object is handed to f( ).

In this case, automatic type conversion has saved you from the trouble of defining two overloaded versions of f( ). However, the cost is the hidden constructor call to Two, which may matter if you’re concerned about the efficiency of calls to f( ).

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire